A conventional air bag device will now be described with reference to the accompanying drawings. FIG. 1 is a sectional view, in generally schematic form, of an air bag installation, and FIG. 2 is a perspective view of a base plate for the air bag. Numeral 1 is a part of a vehicle body, such as a steering wheel, on which an air bag device 3 is mounted on a bracket 2. The air bag device 3 includes a base plate 4, and disposed in an opening 5a on a surface 4a of the base plate 4 is an inflator 5. Further, the air bag device 3 comprises an air bag 6 which is placed in a folded condition and connected to the base plate 4 around the opening 5a so as to contain the inflator 5 in the air bag 6. The air bag device 3 also includes an air bag cover 7 which is connected to the base plate 4 so as to enclose the air bag 6. The base plate 4 is provided with four peripheral walls or flanges 4b having a plurality of holes 8 for receiving bolts that fasten it to the bracket 2.
At the time of the vehicle crash, the gas is introduced into the air bag 6 from the inflator 5, whereby the air bag 6 is inflated so as to pierce the cover 7. The air bag device 3 is mounted in a recessed or set-in position within the steering wheel 9 and upon inflation of the air bag prevents the passenger from colliding directly with the steering wheel.
As described above, the air bag is inflated by the gas jetting from the inflator. At the moment the air bag has ended inflation and reaches its maximum volume, an event that is hereinafter called "the end of development," a very high tensile stress is applied to a cloth material of the air bag. The inflation of the air bag stops rapidly at the end of development, and the inertia force and gas inflation pressure applied to the air bag cloth cause very high tensile stresses in the bag fabric. Therefore, a strong impact is imposed on the whole area of the air bag at the end of development.
To sustain the high forces, it is necessary to use very strong cloth for the air bag and to provide very strong stitching in its construction. In addition to high costs for material and labor, the sewn air bag is confronted with the following problems:
(1) At the end of development of the air bag, each space between adjacent stitches is large. PA1 (2) The sewn bag employs a cloth coated with a rubber. To increase the strength of the seams, the hardness of the coated rubber is increased, so that the rubber-coated cloth is quite stiff. The stiffness of the bag prevents it from being folded into a desirably small volume. PA1 (3) When the hardness of the rubber-coated cloth is high, there is the danger that the coated rubber may be cracked or stripped in case the bag is inflated when it is at a low temperature.
Also, with a conventional air bag device, there is the disadvantage that the steering wheel is deflected by the bag at the end of development of the air bag at the time of the vehicle crash, such that its ability to deflect further when the vehicle occupant collides with the bag is diminished. More specifically, as shown in FIG. 1, a top surface of the steering wheel 9 is spaced by a distance W from a top surface of the air bag device 3. Therefore, at the end of development, the bag is inflated, as shown by a dash-and-dotted line 6', and a force F causes the steering wheel 9 to be deflected forwardly. Accordingly, prior to impact of the occupant against the air bag 6, the steering wheel support arms 9a are deformed by the force F, so that the elastic deforming force of the arms 9a is not available to absorb the energy of the occupant's impact against the air bag.
As a result of a reaction of the force F, a high stress is applied to the structures that join the bag to the base plate and the base plate to the bracket 2. For example, a high force H is applied to the base plate 4 in a direction tending to separate the base plate 4 from the vehicle body 1. For this reason the base plate and the fastening structures must be of high-strength design, which makes them costly.